Method and apparatus for determining optical characteristics of a cornea

Abstract

A system for determining the birefringent topography of a birefringent sample (e.g. cornea of an eye) includes a scanning tomopgraphy unit for establishing a plane of focus and an ellipsometer for generating a laser beam that is useable to obtain a birefringent measurement of the sample. The system also includes a topography unit for determining the angle the laser beam is incident on the sample, and a computer for correcting the birefringent measurement to account for this angle of incidence. A Z-tracker unit is used to maintain a proper relationship between system components during its operation. In a refinement of the system, where the sample is the cornea of an eye, the affects of patient heart beat and respiration can be preprogrammed into the computer for use in correcting the birefringent measurement.

What is claimed is: 
     
       1. A system for determining the birefringent topography of a birefringent sample, the sample having an anterior surface separated from a posterior surface, wherein said system comprises: a scanning tomography unit for establishing a plane of focus within the sample;   an ellipsometer for generating a laser beam to obtain a birefringent measurement of the sample at a point in said plane of focus;   a topography unit for determining an angle of incidence of said laser beam relative to the anterior surface of the sample; and   computer means for revising said birefringent measurement with said angle of incidence to obtain a corrected birefringent measurement.   
     
     
       2. A system as recited in claim 1 further comprising a Z-tracker unit for referencing a base datum for said system relative to said sample, and for measuring a deviation of the anterior surface of the sample from said base datum to generate an error signal, and wherein said computer means minimizes said error signal to maintain said system substantially on said base datum. 
     
     
       3. A system as recited in claim 2 wherein said computer means uses said deviation to refine said angle of incidence. 
     
     
       4. A system as recited in claim 1 wherein said ellipsometer is operated to selectively generate a plurality of said laser beams, each said beam being directed to a respective said plurality of points in said plane of focus to obtain a plurality of corrected birefringent measurements. 
     
     
       5. A system as recited in claim 4 wherein said scanning tomography unit is operated to selectively establish a plurality of planes of focus within the sample. 
     
     
       6. A system as recited in claim 5 wherein said computer means processes said plurality of corrected birefringent measurements from said plurality of planes of focus to create an optical image of the sample. 
     
     
       7. A system as recited in claim 1 wherein said birefringent sample is a cornea of an eye. 
     
     
       8. A system as recited in claim 1 wherein said birefringent topography is obtained during sculpting of the anterior surface of said sample, and wherein said topography unit determines a corneal topography, and said system further comprises: an active mirror;   electronic means interconnecting said topography unit with said active mirror for reconfiguring said active mirror to calibrate the corneal topography on said active mirror; and   computer means for evaluating changes in said active mirror to monitor changes in said corneal topography.   
     
     
       9. A system as recited in claim 1 wherein said scanning tomography unit comprises: a diode laser source for establishing said plane of focus; and   focal means for varying said plane of focus between minus twelve diopters and plus twelve diopters (-12 and +12 diopters) in increments of one quarter diopter (0.25 diopter).   
     
     
       10. A system as recited in claim 9 wherein said diode laser source operates at a wavelength of six hundred and seventy nanometers (670 nm). 
     
     
       11. A system as recited in claim 1 wherein said ellipsometer comprises: a polarizing unit for generating said beam of light having a preselected irradiation state;   an analyzing unit for receiving a reflection of said beam of light, said analyzing unit using a preselected detection state to determine a light intensity state of said beam; and   electronic processor means in said computer means for concertedly varying said polarization state of said polarizing unit with said detection state of said analyzing unit to determine a plurality of said intensity states for said beam for use in obtaining said birefringent measurement.   
     
     
       12. A method for determining the birefringent topography of a birefringent sample, the sample having an anterior surface separated from a posterior surface, wherein said method comprises the steps of: establishing a plane of focus within the sample;   providing a system including an ellipsometer for generating a laser beam to obtain a birefringent measurement of the sample at a point in said plane of focus, and a topography unit for determining an angle of incidence of said laser beam relative to the anterior surface of the sample;   referencing a base datum, said base datum being taken relative to the sample;   refining said angle of incidence according to movement of said base datum; and   revising said birefringent measurement with said refined angle of incidence to obtain a corrected birefringent measurement.   
     
     
       13. A method as recited in claim 12 wherein said referencing step comprises the steps of: measuring a deviation of the anterior surface of the sample from said base datum to generate an error signal; and   minimizing said error signal to maintain said system substantially fixed on said base datum.   
     
     
       14. A method as recited in claim 12 further comprising the step of selectively generating a plurality of said laser beams, each said beam being directed to a respective said plurality of points in said plane of focus to obtain a plurality of corrected birefringent measurements. 
     
     
       15. A method as recited in claim 14 wherein said scanning tomography unit is operated to selectively establish a plurality of planes of focus within the sample, and wherein said computer means processes said plurality of corrected birefringent measurements from said plurality of planes of focus to create an optical image of the sample. 
     
     
       16. A method as recited in claim 12 wherein said birefringent sample is a cornea of an eye. 
     
     
       17. A method as recited in claim 16 further comprising the step of selecting positions on the cornea for transverse cuts based on said corrected birefringent measurements. 
     
     
       18. A method as recited in claim 16 further comprising the steps of: taking corrected birefringent measurements to determine internal stresses of a recipient cornea bed;   taking corrected birefringent measurements to determine internal stresses of a donor corneal button; and   orienting said donor corneal button in said recipient cornea bed in accordance with said respective corrected birefringent measurements.   
     
     
       19. A method as recited in claim 16 further comprising the step of monitoring relaxation of internal stress distribution based on corrected birefringent measurements taken before and after surgery. 
     
     
       20. A method as recited in claim 16 further comprising the step of recording internal stress redistribution due to wound healing based on corrected birefringent measurements. 
     
     
       21. A system as recited in claim 12 wherein said birefringent sample is a cornea of an eye. 
     
     
       22. A system for determining the birefringent topography of a birefringent sample, the sample having an anterior surface separated from a posterior surface, wherein said system comprises: a laser means for establishing a plane of focus within the sample;   a laser polarizer means for generating a laser beam to obtain a birefringent measurement of the sample at a point in said plane of focus;   a reflection analyzing means for determining an angle of incidence of said laser beam relative to the anterior surface of the sample;   a tracking means for referencing a base datum, said base datum being taken relative to the sample; and   a computer means for refining said angle of incidence according to movement of said base datum, and for revising said birefringent measurement with said refined angle of incidence to obtain a corrected birefringent measurement.   
     
     
       23. A system as recited in claim 22 wherein said tracking means measures a deviation of the anterior surface of the sample from said base datum to generate an error signal, and said computer means minimizes said error signal to maintain said system substantially fixed on said base datum. 
     
     
       24. A system recited in claim 22 further wherein said laser polarizer means is an ellipsometer, and said ellipsometer selectively generates a plurality of said laser beams, each said beam being directed to a respective said plurality of points in said plane of focus to obtain a plurality of corrected birefringent measurements. 
     
     
       25. A system as recited in claim 24 wherein said laser means is a scanning tomography unit, and wherein said scanning tomopgaphy unit is operated to selectively establish a plurality of planes of focus within the sample, and wherein said computer means processes said plurality of corrected birefringent measurements from said plurality of planes of focus to create an optical image of the sample.